The expression of a desired protein by gene delivery depends on the cellular processes of DNA transcription and mRNA translation. The desired protein is typically selected from among proteins capable of stimulating the cellular immune system, and the increased protein expression induces cellular immune responses, thus providing therapeutic effects for various diseases. For this, many studies have been conducted for the delivery of plasmid DNA carrying genetic information. However, plasmid DNA has some barriers as follows. Once plasmid DNA enters cells, it must be transported to the nucleus where it can be transcribed into mRNA, and the resulting transcript needs to be exported from the nucleus into the cytoplasm to be translated into a target protein. This multistep process markedly reduces the effectiveness of gene expression, and moreover, plasmid DNA has low efficiency in penetrating through the nuclear envelope. Besides, there is the risk of genomic integration. Newly dividing cells during cell division are apt to contain the delivered exogenous DNA, which can alter the host cell's inherent genetic information. Thus, in order to overcome the problems associated with the delivery of plasmid DNA, many techniques have been developed including the employment of coating materials such as cationic polymers or lipids for particle formation, for example, as described in the following reference: R. Tachibana, H. Harashima, Y. Shinohara, H. Kiwada, Adv. Drug Delivery Rev. 2001, 52, 219-226.
However, another obstacle for DNA delivery is that most of the available organic and inorganic materials are foreign materials which, when taken up by cells, are recognized as foreign materials and thus potentially cause cytotoxicity.